Rotational molding or rotational casting, more commonly known as rotomolding, is widely used for molding hollow articles, and can be used to mold both small and large containers, such as tanks of typically 19 L to 57,000 L. Such rotomolded tanks are utilized in agricultural, chemical and recreational vehicle industries. Rotomolded containers are used for packaging and material handling, particularly as container articles for fluids or solids. Rotational molding is also used for portable toilets, instrument and battery cases, light globes, vacuum cleaner and scrubber housings, toys and garbage containers. The process is relatively less expensive and easy to use for polymer processing than other known means and has been increasing in use.
To rotomold a part, a powdered, polymeric resin is charged inside a mold shell, which is then, typically, rotated on two axes and heated to cause the melting resin to adhere to the inside of the mold. After sufficient heating time, the mold is moved to a cooling chamber, and after cooling, the molded part is removed to begin another molding cycle. More detailed discussion of rotomolding may be found in Modem Plastics Encyclopedia 1990, pages 317-318, and in Encyclopedia of Polymer Science and Engineering, pages 659-670 (J. Wiley & Sons, 1990).
Rotational molding primarily uses polyolefin resins, with thermoplastic polymers of ethylene being principally used. Key properties for rotationally molded parts include appearance, and especially in the case of containers, resistance to puncture or rupture, chemical resistance and for extended periods of usefulness, resistance to environmental stress cracking. Low density polyethylene (LDPE) with a density of about 0.900 to about 0.925 g/cm3, linear low density polyethylene (LLDPE) with a density of about 0.926 to about 0.940 g/cm3, and high density polyethylene (HDPE) with a density of about 0.940 to about 0.960 g/cm3 are used in rotomolding applications. LLDPE is said to be preferred for its excellent low temperature impact strength and good environmental stress crack resistance (“ESCR”).
Compositions of polyethylene resins have been proposed to improve physical properties, including impact strength, environmental stress crack resistance, and chemical resistance. U.S. Pat. No. 4,438,238 describes blends for extrusion processing, injection molding and films where a combination of two ethylene-α-olefin copolymers with different densities, intrinsic viscosities and number of short chain branching per 1000 carbon atoms is attributed with such physical properties. U.S. Pat. No. 4,461,873 describes ethylene polymer blends of a high molecular weight ethylene polymer, preferably a copolymer, and a low molecular weight ethylene polymer, preferably an ethylene homopolymer, for improved film properties and environmental stress crack resistance useful in the manufacture of film or in blow molding techniques, the production of pipes and wire coating. EP 0 423 962 describes ethylene polymer compositions particularly suitable for gas pipes said to have improved environmental stress cracking resistance comprising two or more kinds of ethylene polymers different in average molecular weight, at least one of which is a high molecular weight ethylene polymer having an intrinsic viscosity of 4.5 to 10.0 dl/g in decalin at 135° C. and a density of 0.910 to 0.930 g/cm3 and another of which is a low molecular weight ethylene polymer having an intrinsic viscosity of 0.5 to 2.0 dl/g, as determined for the first polymer, and a density of 0.938 to 0.970 g/cm3.
U.S. Pat. No. 5,082,902 describes blends of linear polyethylenes for injection and rotational molding said to have reduced crystallization times with improved impact strength and ESCR. The blends comprise (a) a first polymer having a density of from 0.85 to 0.95 g/cm3 and an MI of 1 to 200 g/10 min, and (b) a second polymer having a density of 0.015 to 0.15 g/cm3 greater than the density of the first polymer and an MI differing by no more that 50% from the MI of the first polymer. U.S. Pat. No. 5,306,775 describes polyethylene blends said to have a balance of properties for processing by any of the known thermoplastic processes, specifically including improved environmental stress crack resistance. These compositions have (a) low molecular weight ethylene resins made using a chromium oxide based catalyst and having a density at least 0.955 g/cm3 and melt index (MI) between 25 and 400 g/10 min and (b) high molecular weight ethylene copolymer resins with a density not higher than 0.955 g/cm3 and a high load melt index (HLMI) between 0.1 and 50 g/10 min.
U.S. Pat. No. 5,382,631 describes linear interpolymer polyethylene blends having narrow molecular weight distribution (Mw/Mn≦3) and/or composition distribution (CDBI) less than 50%, where the blends are generally free of fractions having higher molecular weight and lower average comonomer contents than other blend components. Improved properties for films, fibers, coatings, and molded articles are attributed to these blends. In one example, a first component is an ethylene-butene copolymer with a density of 0.9042 g/cm3, Mw/Mn of 2.3, and an MI of 4.0 dg/min and a second component is an HDPE with a density of 0.9552 g/cm3, Mw/Mn of 2.8, and an MI of 5.0 dg/min. The blend is said to have improved tear strength characteristics.
U.S. Pat. No. 6,362,270 describes thermoplastic compositions said to be especially suited to rotomolding applications comprising (a) a majority component that may be an ethylene interpolymer having a density greater than 0.915 g/cm3 and preferably a melt index of from about 2 to 500 g/10 min, and (b) an impact additive that may be an ethylene interpolymer having a density less than 0.915 g/cm3 and melt index preferably greater than 0.05 g/10 min and less than 100 g/10 min. Improved physical properties as ascribed to these compositions include improved impact strength and good ESCR.
There is a continuing need for polyethylene-based compositions of improved environmental stress crack resistance and impact strength, particularly for those that are suitable for rotomolding applications.